Japanese Patent No. 2874300 and Japanese Utility Model Registration Application Laid-open No. 48-54702 disclose a piston pump or a piston motor that includes two groups of axial piston cylinders arranged on radially inner and outer sides. Either of these employs an incompressible fluid such as oil as the working medium, the groups of axial piston cylinders on the radially inner and outer sides are arranged with their phases displaced circumferentially, and in the former case the piston diameter of the group of axial piston cylinders on the radially inner side is smaller than the piston diameter of the group of axial piston cylinders on the radially outer side.
Furthermore, Japanese Patent Application Laid-open No. 2000-320453 discloses an expander in which a group of axial piston cylinders and a group of vanes are arranged respectively on the radially inner side and the radially outer side of a rotor, and supplying high-temperature, high-pressure steam to the group of vanes via the group of axial piston cylinders converts pressure energy into mechanical energy.
Expanders employing high-temperature, high-pressure steam as the working medium can be divided into a vane type in which a rotor slidably supporting a vane is disposed within a cam ring, a radial type in which a plurality of cylinders and pistons are arranged radially relative to an axis, and an axial type in which a plurality of cylinders and pistons are arranged parallel to an axis.
Although the vane type expander has the advantage that a high steam expansion ratio can be obtained, a long sealing length between the tip of the vane and the inner periphery of the cam ring is required relative to the volume, and since sealing is difficult, there is a large amount of steam leakage, which is a problem.
In the radial type expander, since the cylinders and the pistons are arranged radially relative to the axis, not only does a fan-shaped dead space formed between adjacent cylinders cause an increase in the dimensions, but is also, if a sliding surface of a rotary valve for distributing the steam among the cylinders is cylindrical and a sliding clearance is provided, there is the problem of an increase in the amount of steam leakage compared with a rotary valve having a flat sliding surface.
In contrast, since the axial type expander has its cylinders and pistons arranged in the axial direction, dead space between the cylinders can be made small and the layout on a radial cross section can be made compact, and the dimensions thereof can be made smaller than the radial type expanders where the dead space is large. Furthermore, the amount of steam leaking between the cylinders and the pistons is smaller than the amount of steam leaking between the vane and the cam ring and, moreover, since it is possible to employ a rotary valve having a flat sliding surface and low leakage of steam, a higher output can be achieved compared with the vane type or radial type expanders.